In recent years, much higher recording density is expected in magnetic disks. In order to meet this requirement, extensive studies have been made on the surface topography of the substrate used in magnetic disks. For example, patent document 1 proposes a condition in which the amplitude of the surface waviness of a wave-length from 0.3 mm to 3.0 mm is made 50 nm or less, and the surface roughness is modified to Ra=2 nm to 7 nm, and Rmax=10 nm to 60 nm. Also, patent document 2, for example, proposes a magnetic disk in which the microwaviness of the surface has a cycle of 0.1 mm to 5 mm, and amplitude of 0.1 nm to 1 nm.
However, in the arts described in patent documents 1 and 2, there are instances where the magnetic head collides with the irregularities on the surface of the substrate when the magnetic head moves thereover, and these could not sufficiently comply with the above-mentioned requirement of high recording density.
Furthermore, patent document 3 describes a glass substrate in which, letting the height of the waviness, which generates on the surface of the substrate, measured by using an interferometer for versatile disks at a measuring wave-length (λ) of 0.4 mm to 5.0 mm be Wa (nm), and letting the height of the microwaviness, which generates on the waviness, measured by using a microscopy for three-dimensional surface-structural analysis at a measuring wave-length (λ) of 0.2 mm to 1.4 mm be NRa (nm), Wa is 0.8 or less, and the ratio of NRa to Wa (NRa/Wa) is 1/1.5 or less if Wa is 0.4 or less; ½ or less if Wa is greater than 0.4 to 0.5; and ⅓ or less if Wa is greater than 0.5 to 0.8.
Patent Document 1: Japanese Unexamined Patent Application, Publication No. H6-96436
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2000-207733
Patent Document 3: Japanese Unexamined Patent Application, Publication No. 2003-223711